Method of improving the corrosion resistance of substrates

ABSTRACT

THE CORROSION RESISTANCE OF SUBSTRATES HAVING ALUMINUM, NICKEL, IRON-TIN ALLOY AND CHROMIZED METALLIC SURFACES IS INCREASED MARKEDLY BY ELECTROPLATING THEREON A THIN COATING OF METALLIC CHROMIUM HAVING A THICKNESS OF ABOUT 0.1 0.6 MICROINCH. IN A PREFERRED VARIANT CORROSION RESISTANT FERROUS METAL SUBSTRATES ARE PROVIDED BY APPLYING A COMPOSITE PROTECTIVE COATING WHICH INCLUDES A SYNERGISTIC COMBINATION OF AN ALUMINUM. NICKEL, IRON-TIN ALLOY OR CHROMIZED COATING AND ABOUT 0.1-0.6 MICROINCH OF ELECTRODEPOSTED METALLIC CHROMIUM AS AN OVERCOATING. THE CORROSION RESISTANCE OF THE FOREGOING CHROMIUM COATED SUBSTRATES MAY BE ENHANCED BY ELECTROCHEMICALLY APPLYING THREOVER A FURTHER CAOTING WHICH IS A MIXTURE OF METALLIC CHROMIUM AND CHROMIUM OXIDE. THE METALLIC CHROMIUM ELECTROPLATED AND/OR ELECTROCHEMICALLY TREATED SUBSTRATES ARE EXCELLENT BASES FOR PROTECTIVE ORGANIC COATINGS, AND PROTECTIVE ORGANIC COATINGS MAY BE APPLIED THERETO TO FURTHER INCREASE THE CORROSION RESISTANCE AND/OR FOR DECORATIVE PURPOSES. THE CORROSION RESISTANT SUBSTRATES PREPARED BY THE METHOD OF THE INVENTION ARE ALSO PROVIDED.

United States Patent Ofiice 3,838,024 METHOD OF HVIPROVING THE CORROSION RESISTANCE F SUBSTRATES Lowell W. Austin, Weirton, W. Va., Louis C. Beale, .Ir., Grosse Ile, Micln, and Edwin J. Smith, Winterville, Ohio, assignors to National Steel Corporation No Drawing. Continuation-impart of abandoned application Ser. No. 822,739, May 7, 1969. This application May 31, 1972, Ser. No. 258,420

Int. Cl. C23b 5/06 US. Cl. 204-41 19 Claims ABSTRACT OF THE DISCLOSURE The corrosion resistance of substrates having aluminum, nickel, iron-tin alloy and chromized metallic surfaces is increased markedly by electroplating thereon a thin coating of metallic chromium having a thickness of about 0.1- 0.6 microinch. In a preferred variant, corrosion resistant ferrous metal substrates are provided by applying a composite protective coating which includes a synergistic combination of an aluminum, nickel, iron-tin alloy or chromized coating and about 0.1-0.6 microinch of electrodeposited metallic chromium as an overcoating. The corrosion resistance of the foregoing chromium coated substrates may be enhanced by electrochemically applying thereover a further coating which is a mixture of metallic chromium and chromium oxide. The metallic chromium electroplated and/or electrochemically treated substrates are excellent bases for protective organic coatings, and protective organic coatings may be applied thereto to further increase the corrosion resistance and/ or for decorative purposes. The corrosion resistant substates prepared by the method of the invention are also provided.

RELATED APPLICATION This application is a continuation-in-part of application Ser. No. 822,739, filed May 7, 1969 for Method of Improving the Corrosion Resistance of Substrates Having Aluminum, Nickel, Iron-Tin Alloy and Chromized Metallic Surfaces and the Corrosion Resistant Substrates Thus Produced and now abandoned.

BACKGROUND OF THE INVENTION Unprotected ferrous metal substrates rust very rapidly when exposed to moisture and the atmosphere and protective coatings are often applied to increase the corrosion resistance. A wide variety of protective coatings have been proposed heretofore including organic coatings such as paints, varnishes and enamels, and coatings of metals such as tin, zinc, aluminum, nickel and chromium.

As a general rule, the metallic coatings are relatively expensive to apply by the prior art processes in thicknesses providing optimum protection, and efforts have been made heretofore to reduce the cost by applying thin protective coatings of aluminum, flash nickel, irontin alloy and chromized ferrous metals. However, when these thin coatings are applied individually, only a relatively low degree of protection is imparted to the ferrous metal substrate and it is not suitable for use in corrosive environments. There was no entirely satisfactory process available heretofore for markedly enhancing the corrosion resistance of ferrous metal substrates having aluminum, flash nickel, iron-tin alloy or chromized ferrous metal coatings thereon whereby satisfactory protection against corrosion may be obtained without detracting from the appearance.

Relatively heavy metallic chromium coatings have been applied heretofore to ferrous metal and other substrates, but the electrodeposition of metallic chromium coatings one microinch or more in thickness has not proved to be 3,838,024 Patented Sept. 24, 1974 entirely satisfactory. For instance, electroplating metallic chromium is very inefficient in current usage and it is not possible to achieve an efiiciency of more than about 2025%. Heavy electrical equipment must be used to provide the large amount of current that is required for electroplating thick metallic chromium coatings, and expensive cooling apparatus is necessary to maintain a suitable bath temperature. Even when employing heavy electrical equipment and extensive cooling capacity, it is possible to achieve only relatively slow line speeds as otherwise the current and cooling capacity requirements are too great. Thus, the protection of ferrous metal and other metallic surfaces by application of heavy metallic chromium coatings in high speed electroplating lines has not met with commercial success prior to the present invention due to the deficiencies in the processing limitations and the equipment presently available.

When metallic chromium coatings having thicknesses greater than one microinch are electrodeposited on ferrous metal and other metallic substrates, the heavy coatings thus produced tend to be highly stressed and to become microcracked on the fiat surface and fail to protect the underlying surface at the sites of the cracks. Also, heavy metallic chromium coatings are milky or cloudy and thus are not entirely satisfactory in appearance. In instances where it was attempted to overcome these problems by applying only an extremely thin metallic chromium coating to ferrous metal substrates, the resulting substrates were not sufliciently corrosion resistant for use in many applications.

It is an object of the present invention to provide a novel method of increasing the corrosion resistance of substrates having aluminum, nickel, iron-tin alloy and chromized metallic surfaces by application of a thin coating of electrodeposited metallic chromium which overcomes the above mentioned disadvantages of the prior art.

It is a further object to provide a novel method of enhancing the corrosion resistance of ferrous metal substrates by application of a composite protective coating which includes an aluminum, nickel, iron-tin alloy or chromized ferrous metal undercoating and a thin coating of electrodeposited metallic chromium as an overcoating.

It is still a further object to apply an additional coating which is a mixture of electrochemically deposited metallic chromium and chromium oxide over the thin metallic chromium coated substrates of the invention to further enhance the corrosion resistance.

It is still a further object to apply a protective and/or decorative organic coating on the metallic chromium electroplated and/or electrochemically treated substrates prepared by the method of the invention.

It is still a further object to provide the various products produced in accordance with the method of the invention.

Still other objects of the invention and the advantages thereof will be apparent to those skilled in the art upon reference to the following detailed description, and the examples.

DETAILED DESCRIPTION OF THE INVENTION INCLUDING PREFERRED VARIANTS THEREOF In accordance with the present invention, the corrosion resistance of substrates having aluminum, nickel, iron-tin alloy and chromized metallic surfaces is increased by electrodepositing thereon a thin coating of metallic chromium having a thickness of about 0.1-0.6 microinch. If desired, the corrosion resistance of the metallic chromium electroplated substrates may be further increased by electrodepositing thereon a coating containing a mixture of metallic chromium and chromium oxide.

Any suitable prior art chromium plating bath may be used for electrodepositing the thin metallic chromium coating. As is well understood in the chromium plating art, chromium plating baths usually contain chromic acid or other suitable water soluble hexavalent chromium compound and a catalyst therefor such as sulfate ion or admixtures thereof with fluoride ion and/ or silicofluoride ion. The chromium compound may be added to the plating bath as chromic acid or as an alkali metal chromate such as sodium chromate, sodium dichromate, potassium chromate or potassium dichromate. The catalyst may be added to the plating bath as the free acid or as a water soluble salt thereof providing the desired ion, such as sulfuric acid or water soluble alkali metal sulfates, fluorides and/ or silicofluorides.

The preferred chromium compound is chromic acid in many instances. The mole ratio of chromic acid to the catalyst is usually about 100:1, but other ratios conventional in the art may be employed such as about 50:1- 20021. The chromic acid content of the bath may range from about 100-400 grams per liter, and is preferably about 150-300 grams per liter. The current density may be the same as is used in prior art chromium plating such as about 300-2000 amperes per square foot and is preferably about 500-1000 amperes per square foot. The bath temperature also may be in accordance with the prior art, and may be about 90-150 F. and is preferably about 100-130 F.

Insoluble anodes are usually used. A composite steel anode prepared by applying a lead coating to the side facing the substrate to be plated and polyvinyl chloride or other inert insulating substance to the opposite side to reduce stray currents is often preferred. The substrate surface to be electroplated with the flash chromium coating is preferably free of contaminants such as oil, dirt, and other substances. When contaminants are present, the substrate surface may be scrubbed free of the same in an aqueous bath, but preferably without employing an acidic or alkaline bath which will attack the surface to an undesirable extent. Examples of still other chromium plating baths and operating conditions are disclosed in US. Pats. Nos. 1,942,469, 2,177,392 and 2,415,724, and in the text Modern Electroplating by Frederick A. Lowenheim, 2nd Edition, John Wiley and Sons, Inc., New York, NY. (1963), the teachings of which are incorporated herein by reference. Chapter 5, pages 80-140, and the references cited on pages 128-140 of Modern Electroplating are especially pertinent.

Regardless of the specific prior art chromium plating bath and operating conditions which are employed, the thin coating of metallic chromium has a thickness of about 0.1-0.6 microinch. Unsatisfactory results are usually obtained due to insufficient metallic chromium in the coating at thicknesses less than about 0.1 microinch. Metallic chromium coatings having thicknesses above 0.6 microinch do not result in an appreciable increase in the degree of corrosion protection that is produced in the synergistic combination, and sometimes tend to be cloudy or milky which detracts from the appearance. Also, heavy metallic chromium coatings tend to be highly stressed and are often microcracked on the surface, and the substrate surface is subject to corrosion at the sites of the cracks. Additionally, coatings above 0.6 microinch are expensive to apply and increase the cost of producing the composite coatings very substantially. Metallic chromium coatings having a thickness of 0.2-0.6 microinch and preferably 0.3-0.4 micro inch are very satisfactory. Preferred results are obtained in many instances with very thin metallic chromium coatings having a thickness of about 0.1-0.3 microinch.

While the electrodeposition of 0.1-0.6 microinch of metallic chromium on substrates having aluminum, nickel, iron-tin alloy and chromized metallic surfaces markedly increases the corrosion resistance and is satisfactory for most applications, nevertheless in certain instances it may be desired to impart still additional corrosion resistance. This may be conveniently accomplished by electrochemically treating the metallic chromium coated substrate as a cathode in an aqueous electrolyte containing a water soluble hexavalent chromium compound to deposit thereon a film or coating containing a combination or mixture of metallic chromium and chromium oxide.

The bath for the electrochemical treatment may contain the same ingredients that are employed in the metallic chromium plating bath but in lower concentrations. For example, the metallic chromium plating bath may be diluted with about 3-7 volumes and preferably about 5 volumes of water to prepare a satisfactory electrochemical treatment bath. The electrochemical treatment bath may contain a water soluble hexavalent chromium compound such as chromic acid, alkali metal chromates including sodium and potassium chromate, alkali metal dichromates including sodium and potassium dichromates and other hexavalent Water soluble chromium salts, and a water soluble catalyst such as the sulfate compounds and admixtures thereof with fluoride and silico-fluoride compounds as previously described for the metallic chromium plating bath. Chromic acid is usually preferred as the hexavalent chromium compound, and satisfactory electrochemical treatment baths may contain about 20- 50 grams per liter and preferably about 35 grams per liter of chromic acid, or about l-2 ounces per gallon and preferably 1.5 ounces per gallon of potassium dichro-- mate. Sulfate ion may be present in an amount of about 0.05-0.2 gram per liter and preferably about 0.10 gram per liter, and silicofluoride ion in an amount of about 0.1-1 gram per liter and preferably about 0.3-0.5 gram per liter, and for still better results about 0.4 gram per liter.

The electrochemical treatment may be at a temperature of about -150F. and is preferably at about F. The metallic chromium coated substrate is treated cathodically under current conditions providing about 25-400 amperes per square foot and preferably about 100-300 amperes per square foot over a period of time sufficient to result in treatment with about 25-500 coulombs per square foot of current, and preferably about 75-125 coulombs per square foot. The optimum treatment is often at a bath temperature of 100-120 F. and at a rate of about 200-300 amperes per square foot for a suflicient period of time to provide approximately 100 coulombs per square foot of current, e.g. about 0.30.5 second.

As a general rule, the film deposited during the electrochemical treatment should contain about 0.65 milligrams per square foot of total chromium present in the metallic chromium and in the chromium oxide. The cathodic electrochemical treatment results in the deposition of a mixture of chromium oxide and metallic chromium, and the total chromium is the amount present in both of these substances. The chromium in the chromium oxide portion of the film seems to be largely in the plus 3 valence state, and it is thought to be hydrated Cr O in many instances. Preferably, the film should contain about 1-3 milligrams per square foot of total chromium in the metallic chromium and chromium oxide contents of the film, and for still better results about 0.8-1.5 milligram of total chromium per square foot.

The metallic chromium coating of the invention, with or without the electrochemical treatment, is tightly adherent and it is an excellent base for organic protective coatings without further treatment. Prior art organic coatings of the types usually applied to tinplate or blackplate may be applied directly to the thin chromium electroplated surface or to the electrochemically treated surface. Examples of suitable organic coatings include phenolic, modified phenolic, epoxy, modified epoxy, polyester, modified polyester, vinyl resin, Teflon, and drying oil-based paints, varnishes, lacquers and enamels. Silicone modified polyester enamels give exceptionally good results in many instances. The organic coatings may be applied by prior techniques such as spraying, brushing, roller coating and electrostatic deposition.

The present invention is especially useful in producing synergistic composite coatings on blackplate strip or sheet of the usual gauge employed in the manufacture of tinplate. For instance, blackplate having a weight of 55-90 pounds per base box may be used in practicing the present invention, but heavier or lighter weights may be used.

The ferrous metal substrate to be provided with the composite coating need not be given a special pretreatment prior to applying the initial coating of aluminum, nickel, iron-tin alloy or chromized ferrous metal provided the surface is free of contaminants. However, in most instances contaminants are present and the ferrous metal substrate should be subjected to the prior art pretreatment or cleaning steps normally used in the manufacture of the initially coated substrate.

In instances where aluminum or aluminum alloy coated metallic substrates are employed, they may be prepared by any suitable prior art process. For example, aluminum or aluminum alloys may be deposited on ferrous metal strip by hot dipping, electrodeposition, vapor deposition of aluminum vapor, decomposition of a heat decomposable aluminum compound, cathode sputtering, electrostatic or electrophoretic deposition, etc. The electroplating of ferrous metal strip is disclosed in Pat. No. 3,007,854, the electrostatic deposition of aluminum on ferrous metal strip is disclosed in Pat. No. 3,382,085, cathode sputtering is disclosed in Pat. No. 2,257,411, deposition of the coating metal by contacting a heated substrate with thermally decomposable organic compounds is disclosed in Pats. Nos. 2,772,985, 2,880,115, 2,886,469, 2,898,235 and 2,921,868, and the electrodeposition of aluminum-manganese alloys is disclosed in Pat. No. 3,167,403. The thickness of the metallic aluminum or aluminum alloy coating may be in accordance with prior art practice and may be as disclosed in the above-mentioned references. The coating thickness may be about 0.5-5 mils, and is preferably about 0.81 mil. The preferred substrate is ferrous metal, and especially blackplate sheet or strip of tinplate gauge and quality.

The nickel coated metallic substrates for use in accordance with the invention also may be prepared by prior art practices. For instance, a nickel coating may be applied to blackplate sheet or strip by electrodeposition, immersion in a chemical plating bath, contacting the heated substrate with a thermally decomposable nickel compound, electrostatic deposition and electrophoretic deposition. The thickness of the nickel coating that is applied by these processes may be in accordance with prior art practice. Examples of suitable electroplating baths and techniques are found in the text Electroplating Engineering Handbook, A. Kenneth Graham, Editor, =Reinhold Publishing Corporation, New York, N.Y., 1955, the teachings of which are incorporated herein by reference. The electrodeposited nickel coatings may have a thickness of, for example, less than 100 microinches and preferably about 01-10 microinches. In most instances, it is preferred that a very thin or flash coating of nickel be electrodeposited such as about 0.5-5 microinches.

Chromized ferrous metal substrates for use in accordance with the invention also may be preparedby prior art processes which are well known to the art. One suitable chromizing process is disclosed in the text The Making, Shaping and Treating of Steel, edited by H. E. McGannon, 8th Edition, 1964. Page 939 of this text is especially pertinent as a description of one suitable chromizing process is disclosed. The amount of chromium which is added to the ferrous metal surface and the thickness of the chromized layer may be in accordance with prior art practice, but usually chromized layers of about 1-10 mils and preferably 1-3 mils are used.

Iron-tin alloy coated ferrous metal substrates may be prepared by a number of methods. In one preferred method, blackplate sheet or strip is given an initial coating of tin by electrodeposition, vapor deposition, electrostatic deposition, electrophoretic deposition or other suitable method, and then the tin coated substrate is heated to an alloying temperature such as about 450-650 F. for a suificient period of time to assure that the tin coating is alloyed with the iron in the substrate. The alloy thus formed is usually FeSn or FeSn. Suitable iron-tin alloy coating thicknesses contain one pound or less of tin per base box, i.e. 62,720 square inches of surface area. Preferably, the iron-tin alloy coating should contain about 0.02-0.1 pound of tin per base box. One suitable method of applying an iron-tin alloy coating to ferrous metal substrates is by electrodeposition of a tin coating followed by heat treatment as disclosed in Pats. Nos. 2,643,975 and 3,285,838. It is also possible to apply a mixture of iron and tin to the ferrous metal substrate by electrodeposition or other suitable process, and then heat the substrate to a temperature sufficiently high to produce the iron-tin alloy. The heat treatment and tin content in the alloy may be as set out above. A coating of iron-tin alloy also may be applied to metallic substrates other than ferrous metal, and the irontin alloy coated substrates thus produced may be used in accordance with the invention.

As previously discussed, the initially coated substrate is passed through a prior art chromium plating bath and about 0.1-0.6 microinch of metallic chromium is electrodeposited over the aluminum, nickel, iron-tin alloy or chromized ferrous metal coating. Thereafter, if desired the substrate with the composite coating may be electrochemically treated to deposit a film containing metallic chromium and chromium oxide. The metallic chromium coated substrate thus prepared, with or without electrochemical treatment, may be washed, dried, oiled, and coiled, or it may be given an organic coating in accordance with prior art practice.

The present invention may be combined with prior art electroplating lines or other types of continuous coating lines for applying the initial metal coating. Thus, the ability to electrodeposit the thin chromium overcoating described herein in a continuous high speed line is of importance in commercial operations as the composite coating may be produced by adding the metallic chromium electrodeposition step to prior art high speed coating lines.

The composite coatings are not discolored, and the chromium plated surface remains bright in appearance. The brightness of a matte aluminum surface such as is produced by electrodeposition on blackplate from a fused salt bath, and the brightness of the composite coating produced therefrom may be further improved by subjecting the substrate to a brushing or rolling step or to other suitable methods designed to provide a bright fresh aluminum surface, prior to electrodepositing the metallic chromium coating. The brushing or rolling step need not be drastic, and it may be a skin pass wherein the substrate is reduced about 12% in thickness to thereby produce a fresh, bright, smooth aluminum surface for receiving the flash coating of metallic chromium. The prerolling step is especially important in instances where it is desired to apply a clear organic coating, or for other purposes where maximum brightness of the final product is desired. The brightness of the iron-tin alloy surfaced substrate may be improved by providing a small amount of unalloyed or free tin in the iron-tin alloy coating.

The composite coating exceeds the combined individual corrosion resistances of the initial metallic coating and the metallic chromium coating by several fold when measured by standard corrosion tests, such as the salt fog test and/or the water immersion test. As a general rule, the composite coating will increase the corrosion resistance at least three fold, and often about five fold or more. The

terms aluminum surface and/or nickel surface as used in the specification and claims are intended to embrace articles having a surface subject to corrosion which is composed of aluminum or nickel or predominantly of aluminum or nickel. Examples of materials or articles having an aluminum or nickel surface to be electrodeposited with the flash coating of metallic chromium include articles composed of massive aluminum or nickel and alloys containing predominantly aluminum or nickel, base materials in general and including corrodible metals which are provided with a decorative or protective coating of aluminum or nickel and alloys containing predominantly aluminum or nickel. Aluminum-manganese alloys in accordance with U5. Pat. No. 3,167,403 may be used. In instances where other aluminum or nickel alloys are used, the alloys preferably should contain more than 50% by weight of aluminum or nickel and the remainder prior art alloying elements therefor and incidental impurities.

The present invention is further illustrated by the following specific examples.

EXAMPLE I A coil of aluminum coated blackplate strip 0.02 inch in thickness and having a 0.8 mil coating of aluminum thereon is used in this example. The aluminum coated strip is produced by a prior art process involving the electrostatic deposition thereon of aluminum powder having a mean mass particle size of microns, agglomerating the green particulate aluminum coating by heating at 500 F. in air, roll compacting the agglomerated particulate aluminum coating to produce a continuous coating, and heat treating the continuous aluminum coating at 500600 F. to assure adherence to the strip surface. The fresh aluminum surface is bright and free of contaminants such as dirt, oil and grease, and it is not necessary to scrub the surface prior to electroplating with a flash coating of chromium.

The above aluminum coated blackplate strip is electroplated with 0.3 microinch of metallic chromium using a chromium plating bath containing 150 grams per liter of chromic acid, sodium sulfate in an amount to provide 11.1 grams per liter of sulfate ion, and sodium silicofluoride in an amount to provide 1.8-2.0 grams per liter of silicofluoride ion. The temperature of the electrolyte is 120 F., and the current density is 800-1000 amperes per square foot. The strip is passed through the chromium plater at a speed of 400 feet per minute, and it is not treated prior to passing into the plater with the exception of wetting with water.

The blackplate strip withdrawn from the plater has a composite coating thereon which consists of 0.8 mil of aluminum in contact with the steel basis metal, and 0.3 microinch of metallic chromium over the aluminum coating. The strip is washed to remove the electrolyte, dried, and coiled without oiling. Samples of the blackplate strip having the composite coating thereon are tested by the standard ASTM salt fog corrosion test (ASTM method B-l17) for the periods of time indicated below. Samples of the aluminum coated blackplate strip prior to the electrodeposition of the chromium coating, and blackplate strip electroplated with 0.3 microinch of metallic chromium by a prior art process are tested under the same conditions to provide comparative data.

After only two hours of testing in the salt fog test chamber, red rust is present on 5% of the surface area of the blackplate strip electroplated with 0.3 microinch of metallic chromium. At the end of 336 hours, the surface of the blackplate strip coated with 0.8 mil of aluminum has red rust on 8% of the surface area, and it is also badly pitted. However, at the end of 336 hours the blackplate strip with the composite aluminum-chromium coating has red rust on less than 1% of the surface area, and no pitting. Upon extending the test to 600 hours, there is still no pitting of the blackplate strip coated with the composite coating. Thus, the composite coating exhibits a marked synergistic effect.

The adhesion of the metallic chromium is excellent as shown by deformation tests. Also, it is possible to apply organic protective coatings on the composite coating without further treatment.

EXAMPLE II The general procedure of Example I is repeated with the exception of substituting blackplate strip electroplated by a prior art process with three microinches of metallic nickel for the aluminum coated strip. After electrodepositing 0.3 microinch of metallic chromium over the nickel coating following the procedure of Example I, the strip surface is much improved in appearance and is extremely bright.

Upon testing samples of the blackplate strip electroplated with only 3.0 microinch of metallic nickel following the test procedure of Example I, the surface area of the samples is covered with rust after 16 hours in the salt fog chamber. When samples of the blackplate strip with the nickel-chromium coating of the invention are tested in the salt fog chamber under identical conditions, the samples show no rust thereon after 16 hours. Thus, the nickel-chromium composite coating of the invention also exhibits a marked synergistic effect.

The adhesion of the metallic chromium coating to the initial flash nickel coating is excellent as is shown by deformation tests. Also, it is possible to ap ly organic protective coatings on the composite nickel-chromium coating without further treatment.

EXAMPLE III The general procedure of Example I is repeated with the exception of substituting blackplate strip having an iron-tin alloy layer thereon for the aluminum coated strip. The iron-tin alloy coated blackplate strip is prepared by a prior art process which involves the electrodeposition of a flash coating of tin thereon from an acidic halogen bath in an amount of 0.035 pound per base box, heating the strip at a temperature above the melting point of tin until the flash tin coating is alloyed with the ferrous metal substrate to produce FeSn followed by quenching, drying, and coiling without application of oil. The fresh iron-tin alloy coated blackplate surface thus produced does not require further treatment before electroplating with the flash metallic chromium coating. After electrodepositing the 0.3 microinch of metallic chromium over the iron-tin alloy coated blackplate strip, the surface is bright and much improved in appearance.

Upon testing samples of the blackplate having only the iron-tin alloy coating thereon following the procedure of Example I, the surface area is covered with rust after 16 hours in the salt fog chamber. When samples of the irontin alloy-metallic chromium composite coating of the invention are tested under identical conditions, no rust is apparent after 16 hours. Thus, the iron-tin alloy-metallic chromium composite coating of the invention also exhibits a marked synergistic effect.

The adhesion of the metallic chromium coating to the iron-tin alloy coated blackplate is excellent as is shown by deformation tests. However, it is not possible to electroplate a satisfactory flash chromium coating on the flash tin coating prior to the alloying step, and thus the iron-tin alloy coating is of importance. It is possible to apply organic protective coatings on the iron-tin alloymetallic nickel composite coating without further treatment.

EXAMPLE IV The general procedure of Example I is repeated with the exception of substituting chromized blackplate strip prepared by a prior are chromizing process for the aluminum coated strip. The chromized coating on the black- 9 plate strip is 2 mils in thickness. After electrodepositing 0.3 microinch of metallic chromium over the chromized steel coating following the procedure of Example I, the strip surface is brighter and much improved in appearance.

Upon testing samples of the initial chromized blackplate strip as in Example I, red rust is observed on the surface after only 48 hours in the salt fog chamber. Samples of the blackplate strip coated with the chromized steel-metallic chromium composite coating of the invention show no rusting upon testing under identical conditions for 840 hours. Thus, the chromized steel-metallic composite coating of the invention exhibits a marked synergistic effect.

The adhesion of the metallic chromium coating to the chromized steel coating is shown to be excellent by deformation tests. Also, it is possible to apply organic protective coatings on the chromized steel-metallic chromium composite coating without further treatment.

EXAMPLE V The products produced in accordance with Examples I through IV are given an electrochemical treatment in this example in which a film containing a mixture of metallic chromium and chromium oxide is deposited over the composite coatings.

Blackplate strip product from each of the four examples having the composite coating thereon is passed into an electrochemical treating vessel filled with an electrolyte containing 35 grams per liter of chromic acid, about 0.1 gram of sulfate ion per liter, and about 0.4 gram of silicofluoride ion per liter. The elecroly-te temperature is maintained at about 120 F. and the strip surface is treated cathodically at 250 amperes per square foot for a period of time to provide a surface treatment of 100 coulombs per square foot, i.e., about 0.4 second. A film is deposited on the strip which contains 1-2 mg. per square foot of total chromium. The strip is withdrawn from the electrochemical treating vessel, rinsed with water, dried and coiled.

Samples of the four chemically treated strips thus produced are tested following the procedure outlined in Example I. The test results show that the corrosion resistance of the four chemically treated strips is increased substantially. Thus, electrochemical treatment in accordance with the invention further increases the corrosion resistance of the composite coatings.

EXAMPLE VI The general procedure of Example IV for electroplating metallic chromium on chromized blackplate 18 16 peated in a series of runs with the exception of using chromized blackplate having a chromized coating 2 mils in thickness and electroplating thereon metallic chromium coating weights varying between 0.05 microinch and 5.0 microinches. The products thus prepared are given an electrochemical treatment in accordance with Example V, and then samples are tested following the general procedure outlined in Example I. The samples are observed each 24 hours over the next 12 days, i.e., up to expiration of 288 hours in the salt fog. The data thus obtained are recorded below:

Hours in salt fog 24 48 72 96 120 144 168 192 216 240 264 288 Microinehes of Cr Percent red rust The above data show that the electrochemical treatment of the thin chromium coatings of the invention upgrades the performance of the composite coating very substantially. Surprisingly, in most instances the electrochemically treated thin metallic chromium coatings of the invention provide substantially as much protection as the heavy metallic chromium coatings.

We claim:

1. A method of increasing the corrosion resistance of substrates having metallic surfaces selected from the group consisting of metallic aluminum surfaces, metallic nickel surfaces, iron-tin alloy surfaces and chromized metallic surfaces comprising electroplating at least a portion of the surface area thereof with a coating of metallic chromium having a thickness of about 0.1-0.6 microinch, and thereafter electrochemically treating the resultant metallic chromium coated substrate as a cathode in an aqueous electrolyte containing a water soluble hexavalent chromium compound and a catalyst to deposit a metallic chromium and chromium oxide containing film thereon.

2. The corrosion resistant substrate prepared by the method of claim 1.

3. The method of claim 1 wherein the said metallic chromium and chromium oxide containing film contains a total of 0.6-5 milligrams of chromium per square foot.

4. The method of claim 1 wherein the said metallic chromium and chromium oxide containing film contains a total of about 0.8-1.5 milligrams of chromium per square foot.

5. The method of claim 1 wherein the metallic chromium coating has a thickness of about 0.2-0.6 microinch.

6. The method of claim 1 wherein the metallic chromium coating has a thickness of about 0.1-0.3 microinch.

7. The method of claim 1 wherein the substrate is metallic aluminum coated ferrous metal.

'8. The method of claim 1 wherein the substrate is metallic nickel coated ferrous metal.

9. The method of claim 1 wherein the substrate is irontin alloy coated ferrous metal.

10. The method of claim 1 wherein the substrate is chromized ferrous metal.

11. The method of claim 1 wherein the aluminum surface is metallic aluminum coated ferrous metal, said metallic aluminum coating having a thickness of 0.5-5 mils,

the nickel surface is metallic nickel coated ferrous metal, said nickel coating having a thickness of less than microinches,

the iron-tin alloy surface is iron-tin alloy coated ferrous metal, said iron-tin alloy coating containing not more than one pound of tin per base box, and the chromized metallic surface is chromized ferrous metal, the chromized ferrous metal having a chromized layer thereon having a thickness of 1-10 mils. 12. The method of claim 11 wherein said metallic aluminum coating has a thickness of about 0.8-1 mil,

said metallic nickel coating has a thickness of 0.1-10

microinches,

said iron-tin alloy coating contains 0.02-0.1 pound of tin per base box, and

said chromized layer has a thickness of 1-3 mils.

13. The method of claim 1 wherein the said substrate to be electroplated with the coating of metallic chromium is selected from the group consisting of metallic aluminum coated steel, metallic nickel coated steel, iron-tin alloy coated steel and chromized steel.

14. The method of claim 13 wherein the said substrate to be electroplated with the coating of metallic chromium is metallic aluminum coated steel, and the resulting steel has a composite protective coating thereon including a coating of metallic aluminum adjacent the steel surface, the said electroplated coating of metallic chromium over the metallic aluminum coating, and the 1 1 said metallic chromium and chromium oxide containing film over the metallic chromium coating.

15. The method of claim 13 wherein the said substrate to be electroplated with the coating of metallic chromium is metallic nickel coated steel, and the resulting steel has a composite protective coating thereon including a coating of metallic nickel adjacent the steel surface, the said electrodeposited coating of metallic chromium over the metallic nickel coating, and the said metallic chromium and chromium oxide containing film over the metallic chromium coating.

16. The method of claim 13 wherein the said substrate to be electroplated with the coating of metallic chromium is iron-tin alloy coated steel, and the resulting steel has a composite protective coating including a layer of an iron-tin alloy on the surface of the steel, the said electroplated coating of metallic chromium over the iron-tin alloy layer, and the said metallic chromium and chromium oxide containing film over the metallic chromium coating.

17. The method of claim 13 wherein the said substrate to be electroplated with the coating of metallic chromium is chromized steel, and the resulting steel has a composite protective coating including a chromized layer on the surface of the steel, the said electroplated coating of metallic chromium over the chromized layer, and the said 25 metallic chromium and chromium oxide containing film over the metallic chromium coating.

18. The method of claim 17 wherein the chromized steel has a chromized layer thereon having a thickness of about 1-10 mils, and the said metallic chromium and chromium oxide containing film contains a total of about 0.6-5 milligrams of chromium per square foot.

19. The method of claim 17 wherein the chromized steel has a chromized layer thereon having a thickness of about 1-3 mils, the said electroplated coating of metallic chromium has a thickness of about 0.1-0.3 microinch, and the said metallic chromium and chromium oxide containing film contains a total of about 0.8-1.5 milligrams of chromium per square foot.

References Cited UNITED STATES PATENTS 3,616,303 10/1971 Carter 20441 3,526,486 9/1970 Smith et al. 204-41 3,532,608 10/1970 Serra 20441 3,567,599 3/1971 Carter et al. 20441 3,642,587 2/1972 Allen 20441 JOHN H. MACK, Primary Examiner R. L. ANDREWS, Assistant Examiner US. Cl. X.R. 20456 R 

